map a buffer from Kernel to User space allocated by another module

Question

I am developing a Linux kernel driver on 3.4. The purpose of this driver is to provide a mmap interface to Userspace from a buffer allocated in an other kernel module likely using kzalloc() (more details below). The pointer provided by mmap must point to the first address of this buffer.

I get the physical address from virt_to_phys(). I give this address right shifted by PAGE_SHIFT to remap_pfn_range() in my mmap fops call.

It is working for now but it looks to me that I am not doing the things properly because nothing ensure me that my buffer is at the top of the page (correct me if I am wrong). Maybe mmap()ing is not the right solution? I have already read the chapter 15 of LDD3 but maybe I am missing something?

Details:

The buffer is in fact a shared memory region allocated by the remoteproc module. This region is used within an asymetric multiprocessing design (OMAP4). I can get this buffer thanks to the rproc_da_to_va() call. That is why there is no way to use something like get_free_pages().

Regards

Kev

Answer 1

Yes, you're correct: there is no guarantee that the allocated memory is at the beginning of a page. And no simple way for you to both guarantee that and to make it truly shared memory.

Obviously you could (a) copy the data from the kzalloc'd address to a newly allocated page and insert that into the mmap'ing process' virtual address space, but then it's not shared with the original datum created by the other kernel module.

You could also (b) map the actual page being allocated by the other module into the process' memory map but it's not guaranteed to be on a page boundary and you'd also be sharing whatever other kernel data happened to reside in that page (which is both a security issue and a potential source of kernel data corruption by the user-space process into which you're sharing the page).

I suppose you could (c) modify the memory manager to return every piece of allocated data at the beginning of a page. This would work, but then every time a driver wants to allocate 12 bytes for some small structure, it will in fact be allocating 4K bytes (or whatever your page size is). That's going to waste a huge amount of memory.

There is simply no way to trick the processor into making the memory appear to be at two different offsets within a page. It's not physically possible.

Your best bet is probably to (d) modify the other driver to allocate the specific bits of data that you want to make shared in a way that ensures alignment on a page boundary (i.e. something you write to replace kzalloc).